dear Salas,
thanks for the link and for your efforts to modify the RIAA and publish the NAB EQ curve; I am lucky cause 7 1/2 IPS has the same EQ curve just like the 15 IPS that is published in the schematic, only the 30 IPS NAB curve is different to these

Not really, I took it to 1800 for when the BF245 went so to have ~22/2 across each buffer JFET for design elegance. That's in conjunction with the IDSS of the lower buffer JFET. If you will be rebuilding component quality and is handy try make the voltage 22V after the resistor with proper value judged in your build. If you will be able to discern anything better it can only be a plus. What it practically does is it equalizes the dissipation on the buffer JFETs.